Chapter 10 Quality of Service (QoS)
VMG1312-B Series User’s Guide
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10.8 Technical Reference
The following sect ion cont ains addit ional t echnical inform ation about the Device feat ures descr ibed
in t his chapt er.
IEEE 802.1Q Tag
The I EEE 802.1Q st andard defines an explicit VLAN t ag in t he MAC header t o identify t he VLAN
m em bership of a fram e across bridges. A VLAN t ag includes the 12-bit VLAN I D and 3-bit user
priorit y. The VLAN I D associat es a fram e wit h a specific VLAN and provides t he inform at ion t hat
devices need to process the fram e across the network.
I EEE 802.1p specifies t he user priority field and defines up to eight separat e t raffic types. The
following table describes t he traffic t ypes defined in t he I EEE 802.1d st andard (which incorporates
the 802.1p) .
DiffServ
QoS is used to prioritize source-t o- dest ination t raffic flows. All packets in t he flow are given t he
sam e priority. You can use CoS ( class of service) to give different priorit ies to different packet
ty pes.
DiffServ ( Different iat ed Services) is a class of service ( CoS) m odel that m arks packets so t hat they
receive specific per- hop treatm ent at DiffServ- com pliant net work devices along t he rout e based on
the application t ypes and t raffic flow. Packet s are m arked wit h DiffServ Code Points ( DSCPs)
indicat ing the level of service desired. This allows t he int erm ediary DiffServ-com pliant network
devices t o handle the packets differently depending on the code points wit hout the need t o
negotiat e paths or rem em ber st ate inform at ion for every flow. I n addition, applicat ions do not have
to request a particular service or give advanced not ice of where the t raffic is going.
DSCP and Per-Hop Behavior
DiffServ defines a new Different iated Services ( DS) field to replace the Type of Service ( TOS) field
in the I P header. The DS field cont ains a 2-bit unused field and a 6- bit DSCP field which can define
up t o 64 service levels. The following figure illustrates the DS field.
Table 49 I EEE 802.1p Priorit y Level and Traffic Type
PRIORITY
LEVEL
TRAFFIC TYPE
Level 7 Typically used for networ k control traffic such as router configuration m essages.
Level 6 Typically used for voice t raffic t hat is especially sensitive t o jitt er ( j it ter is the
variations in delay) .
Level 5 Typically used for video t hat consum es high bandwidth and is sensit ive to j it t er.
Level 4 Typically used for cont rolled load, lat ency-sensitive traffic such as SNA ( Syst em s
Net w ork Archit ecture) transact ions.
Level 3 Typically used for “ excellent effort” or bet t er t han best effort and would include
im port ant business traffic that can tolerate som e delay.
Level 2 This is for “ spare bandwidt h”.
Level 1 This is typically used for non-cr it ical “background” traffic such as bulk transfers t hat
are allowed but that should not affect ot her applicat ions and users.
Level 0 Typically used for best- effort t raffic.
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